1. Field of the Invention
The invention relates to a process for the manufacture of plastic plates with very finely structured surface, in particular a process for the manufacture of linear Fresnel lenses and solar collectors containing said Fresnel lenses.
2. Background of the Related Art
From various points of view, technology has developed methods for structuring plastic surfaces, provided the plastic is suitable to this end. For example, in the case of thermoplastics the surface is structured preferably by means of an embossing tool acting on the surface which is in a corresponding temperature state. (Becker-Braun, Polymer Handbook, vol. 1, Hanser Verlag, 1990, pp. 543-544; K. Stockhert, Finishing of Plastic Surfaces, Hanser 1975.) There exists commercially PMMA-based plastic plate material with characteristically structured surfaces. Said surfaces are manufactured, for example through extrusion with simultaneous embossing in a three roll polishing stack (calender). At the same time, one roller (embossing roller) is provided with the negative of the desired plate structure. The goal with structured plates is to obtain an optimally accurate reproduction of the detail of the roller structure. This goal is reached by setting the melt viscosity as low as possible and by setting the roller temperature as high as possible. Furthermore, as known from everyday practice, the pressure maximum upstream of the narrowest point in the nip (i.e. the gap between the polishing and the structured roller) should be high in order to be able to transfer as much embossing force as possible. The results of the three aforementioned conditions are unavoidable compromises in the technical implementation of the extrusion of structured plates.
The manufacture of plastic plates with structured surfaces according to the aforementioned state of the art has reached its limits, especially where especially high demands are imposed on fineness and accuracy of the structure. For example, the possibility of adapting the described boundary conditions is limited. The roller temperature cannot be arbitrarily increased, since most of the polymer melts adhere to hot metals. This tendency to stick results in separating problems at the embossing roller, starting at a certain roller temperature. Moreover, the melt viscosity of the plastic cannot be selected arbitrarily low, for example, by setting high melt temperatures, since otherwise the embossing force in the nip is too low.
The accurate reproduction of the detail of plates that are produced according to this process and with these limitations is--as stated above--not good enough for certain applications, i.e., fine structures are not correctly shaped or they are rounded off. However, such finely shaped surface structures represent a necessary prerequisite for interesting engineering applications.
In many fields of application in engineering optics it is often important to focus the light emitted from a light source. To this end, suitably dimensioned optical lenses are generally used. To produce a parallel pencil of rays, the light source should be in the focal point of the lens. The shorter the distance between light source and lens, the more light is collected. The result in everyday practice is a lens that exhibits a short focal distance and correspondingly a greater thickness.
From A. Fresnel stems the knowledge that there is also a lens effect when concentrically annular "steps" with comparable surface curvature are substituted for the closed convex surface of the lens. Fresnel lenses are thus so-called step lenses. They can be thought of as a normal lens that has been divided into segments and the surface of each segment has undergone a parallel shift (cf. FIG. 3).
Through the application of this principle, it is possible to build a lens with a high refractive power (short focal distance) and small thickness. Fresnel lenses can also be built as circular and as linear (cylinder) lenses. Known engineering applications for Fresnel lenses are, e.g., for projection screens, overhead projectors, filters for stage spot lights, wide angle view lenses for rear windows of motor vehicles such as delivery trucks or mobile homes, lenses for beacons, and the like.